1. Field of the Invention
The present invention relates to a nonvolatile semiconductor storage device and a method for controlling the same, particularly to a nonvolatile semiconductor storage device used in a flash EEPROM (Electrically Erasable and Programmable Read Only Memory) and a method for controlling the same.
2. Related Art
Ordinarily IPD (Inter-Poly Dielectric) breakdown that is a factor degrading reliability of a nonvolatile semiconductor storage device is a problem.
A conventional NAND flash memory is configured such that a dummy cell disposed in an end portion of a memory cell array becomes a floating state in programming operation (see JP-A No. 2008-60421 (Kokai)).
However, when a program-erase cycle is repeated in a memory cell, charges of the dummy cell increase. Therefore, a threshold of the dummy cell is lowered. As a result, the IPD breakdown of the dummy cell is caused.